Activated carbon is a widely used adsorbent in industrial processes to remove contaminants from gas or liquid streams. For example, attempts to meet currently pending mercury emissions limits for fossil fuel fired power plants by injecting powdered activated carbon (PAC) into the flue gas upstream of a particulate control device in order to remove contaminants from the flue gas are being investigated.
The removal of mercury from flue gas streams from combustion processes is of significant interest. The toxicity of mercury to humans has long been known. An example of the devastating effects of mercury exposure occurred in Minamata, Japan in the 1950's where organic mercury byproducts of acetaldehyde production were discharged into the local bay, and were ingested and metabolized by fish. By consuming fish in the bay, wide spread neurological damage and birth defects to the local population were reported.
Coals used for various combustion processes typically contain about 0.1 ppm mercury. In the United States alone, about 50 tons of mercury are discharged as vapor in stack gas every year. Through chemical and biological processes, such mercury can become concentrated by many thousand-fold into fish, thus entering human food supplies at harmful levels. In December 2000, the Environmental Protection Agency (EPA) made its regulatory decision that mercury emissions from coal-fired electric generating plants need to be controlled.
One barrier to the use of adsorbents, however, has been the high cost of both producing and shipping PAC to the end use point. PAC is typically produced from carbonaceous starting materials such as coal, wood, biomass materials, nutshells (e.g., walnut shells, palm nut) or nut hulls (e.g., coconut) that initially do not have high adsorptive characteristics. The carbonaceous starting materials are converted into PAC materials exhibiting higher adsorptive properties by energy and capital intensive processes that include pyrolyzing the feedstock in a rotary kiln, activating the carbon with an activation media (i.e. steam), and grinding or pulverizing the resulting char. The activated carbon material must then be shipped to its end use point, such as a coal-fired power plant.
Attempts to produce PAC near an end use point have been made. For instance, the use of a combustion process to produce char for mercury control has been discussed in the patent literature. U.S. Pat. No. 6,451,094 B1 to Chang, et al. discuss injecting a feedstock into a hot flue gas and activating the feedstock in situ.
U.S. Pat. No. 6,595,147 to Teller et al. relates to adding a carbonaceous char to the flue gas while it is still within a resource recovery unit at a temperature high enough to devolatilize the material to form activated char in situ.
Attempts have also been made to use carbon found in fly ash to capture mercury from flue gas in coal-fired processes. U.S. Pat. No. 5,787,823 to Knowles proposes a method in which carbon-containing fly ash is captured in a cyclone upstream of a conventional particulate control device (PCD). The captured material is then injected into a duct to capture mercury. U.S. Pat. No. 6,027,551 to Hwang, et al. teach separation of the carbon from fly ash captured in a conventional PCD and injection of the carbon-rich portion as a mercury sorbent.
U.S. Pat. No. 6,521,021 B1 to Pennline et al. teach removing partially oxidized coal from the combustion zone of a boiler. The coal is separated out and injected further downstream as a mercury sorbent.
Lanier, et al. (U.S. Pat. No. 6,726,888 B2) suggest controlling the combustion process such that both the NOx emissions and the characteristics of the native fly ash for mercury removal are controlled.
Given both the constraints of normal boiler operation, and the fact that the activity of the native residual carbon decreases as it moves through the boiler, separate production processes for activated carbon can provide a better opportunity to produce activated carbon having characteristics favorable for an intended and particular end use. It would therefore be desirable to provide systems and methods for onsite production of activated carbon suitable for a wide range of processes, thereby improving the cost and efficiency of activated carbon production use.